There are certain words, in any language, that are impossible to translate without compromising their meaning. There are, for example, German words that you can find in your English thesaurus that have been 'adopted' as are in the English language and by now form part of it, like for example, "Zeitgeist", "Weltanschauung" and "Angst".
A "Grübler" is a person that gets caught up in his own stream of negative thoughts. These thoughts are made up of endless-loop questions that by their nature seem impossible to answer. Questions like "Why does nobody like me?", "Why does this always happen to me?", "Why am I such a failure?", "Why can't I please my boss?" etc.
"Grübeln" is as such a pretty dangerous thing to do. We have to take into account here that yes, our brain/mind is like a very powerful computer, but it is at the same time pretty stupid. It doesn't distinguish between what's real and what's not. For your mind, what you think IS reality. It takes everything that goes on in your head at face value, whether your thoughts reflect objective reality or not.
And it doesn't distinguish between a stupid question or a smart question. What's more, any question you ask yourself on a consistent basis, no matter whether that question is grounded in reality or not, will sooner or later be answered by your mind.
Imagine the kind of absurd answers your mind will eventually throw at you in response to the question "Why does nobody like me?" That's pretty simple to imagine, isn't it? "I'm not a lovable person, anyway", "It's always been like this, and will continue to be like this for all eternity", "I suffer from emotional frigidity" and "Well, that's because I smell". Get the picture?
There are some ways to get off this merry-go-round of negative questions.
First of all, become conscious of the process. Observe your mind and what's going on inside. Realize that they are only thoughts, some little storm of neurons making their way through that grey mass we call our brain.
Realize that you are not your thoughts. The questions of who you really are is irrelevant in this context, but you have to admit that if you're able to observe your thoughts, 'you' cannot be 'them'.
If you insist on having negative thoughts, start each one of them with the introduction "I'm having the thought that........ (the world is going to end tomorrow, will I ever get laid again?, I have to declare bankruptcy, etc.) That helps you to distinguish between what's really going on and what's only in your mind.
Do a little visualization exercise. See yourself putting all those negative thoughts on a big leave, put that leaf onto the current of a river, and see it floating away.
And last, but not least, come up with more intelligent questions. Instead of asking "Why does nobody like me? ask yourself "How can I be of value to my family and friends today?" Remember, the same rules apply, and your brain will provide you with an answer.